Method and System for Transporting Inventory Items

ABSTRACT

A system for transporting inventory items includes an inventory holder capable of storing inventory items and a mobile drive unit. The mobile drive unit is capable of moving to a first point with the inventory holder at least one of coupled to and supported by the mobile drive unit. The mobile drive unit is additionally capable of determining a location of the inventory holder and calculating a difference between the location of the inventory holder and the first point. The mobile drive unit is then capable of determining whether the difference is greater than a predetermined tolerance. In response to determining that the difference is greater than the predetermined tolerance, the mobile drive unit is also capable of moving to a second point based on the location of the inventory holder, docking with the inventory holder, and moving the mobile drive unit and the inventory holder to the first point.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 12/890,197filed Sep. 24, 2010 and entitled “Method and System for TransportingInventory Items” which is a divisional application of U.S. applicationSer. No. 11/423,294 filed Jun. 9, 2006 and entitled “Method and Systemfor Transporting Inventory Items”, now U.S. Pat. No. 7,826,919.

TECHNICAL FIELD OF THE INVENTION

This invention relates in general to inventory systems, and moreparticularly, to a method and system for transporting inventory itemswithin an inventory system.

BACKGROUND OF THE INVENTION

Modern inventory systems, such as those in mail-order and e-commercewarehouses, airport luggage systems, and custom-order manufacturingfacilities, face significant challenges in providing fast, accurateresponses to requests for inventory items. In recent years, automationhas improved the speed and efficiency of storing and retrievinginventory items within such systems. However, automated systems can beextremely vulnerable to operational errors. In particular, whencomponents within an automated system deviate from their anticipatedoperation, errors may be introduced. Correcting such errors may requiremanual intervention on the part of a human operator, thereby limitingthe benefits of automation.

SUMMARY OF THE INVENTION

In accordance with the present invention, the disadvantages and problemsassociated with inventory systems have been substantially reduced oreliminated. In particular, a mobile inventory system is provided thatsupports techniques for automatic correction of particular types ofoperational errors.

In accordance with one embodiment of the present invention, a system fortransporting inventory items includes an inventory holder and a mobiledrive unit. The inventory holder is capable of storing inventory items.The mobile drive unit is capable of moving to a first point with theinventory holder at least one of coupled to and supported by the mobiledrive unit. The mobile drive unit is additionally capable of determininga location of the inventory holder and calculating a difference betweenthe location of the inventory holder and the first point. The mobiledrive unit is then capable of determining whether the difference isgreater than a predetermined tolerance. In response to determining thatthe difference is greater than the predetermined tolerance, the mobiledrive unit is also capable of moving to a second point, docking with theinventory holder, and moving the mobile drive unit and the inventoryholder to the first point.

In accordance with another embodiment of the present invention, a methodfor transporting inventory items includes moving a mobile drive unit toa first point while an inventory holder is at least one of coupled toand supported by the mobile drive unit. Additionally, the methodincludes determining a location of the inventory holder and calculatinga difference between the location of the inventory holder and the firstpoint. The method also includes then determining whether the differenceis greater than a predetermined tolerance. In response to determiningthat the difference is greater than the predetermined tolerance, themethod further includes moving the mobile drive unit to a second point,docking the mobile drive unit with the inventory holder, and moving themobile drive unit and the inventory holder to the first point.

Technical advantages of certain embodiments of the present inventioninclude an inventory management system that is easily scalable, that canbe easily adjusted to manage inventory items of varying types, sizes andshapes, and that can be operated with minimal human intervention.Additionally, particular embodiments of the present invention maysupport automated verification and correction of the placement ofinventory holders by mobile drive units and/or other navigationalfeatures that limit the number of errors introduced by the operation ofthe mobile drive units.

Other technical advantages of the present invention will be readilyapparent to one skilled in the art from the following figures,descriptions, and claims. Moreover, while specific advantages have beenenumerated above, various embodiments may include all, some, or none ofthe enumerated advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsadvantages, reference is now made to the following description, taken inconjunction with the accompanying drawings, in which:

FIG. 1 illustrates an inventory storage system according to a particularembodiment;

FIGS. 2A and 2B represents a cross-sectional diagram of a mobile driveunit according to a particular embodiment;

FIG. 3 illustrates the contents of an inventory holder according to aparticular embodiment;

FIGS. 4A-4H show operation of various components of the mobile driveunit and the inventory holder during docking, movement, and undocking;and

FIG. 5 is a flowchart illustrating certain aspects of the operation ofparticular embodiments of mobile drive unit;

FIG. 6 is a flowchart illustrating other aspects of the operation ofparticular embodiments of mobile drive unit;

FIG. 7 is a flowchart illustrating yet other aspects of the operation ofparticular embodiments of mobile drive unit; and

FIG. 8 is a flowchart illustrating yet other aspects of the operation ofparticular embodiments of mobile drive unit.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an inventory system 10 for storing, sorting, andretrieving inventory items 40 that includes a mobile drive unit 20 andan inventory holder 30. Inventory holder 30 stores multiple inventoryitems 40 of various item types. Mobile drive unit 20 moves inventoryholder 30 between designated points within a workspace associated withinventory system 10.

Mobile drive unit 20 is capable of moving within the workspace and mayinclude any appropriate components for propelling itself and navigatingto a particular destination within the workspace. Additionally, mobiledrive unit 20 may dock with inventory holder 30 in any appropriatemanner so that inventory holder 30 is coupled to and/or supported bymobile drive unit 20 when mobile drive unit 20 is docked with inventoryholder 30. When docked to inventory holder 30, mobile drive unit 20 isalso capable of propelling and/or otherwise moving inventory holder 30.Mobile drive unit 20 may include any appropriate components for dockingwith inventory holder 30 and for maneuvering inventory holder 30 whileinventory holder 30 is docked with mobile drive unit 20. The componentsof a particular embodiment of mobile drive unit 20 are described ingreater detail below with respect to FIGS. 2A and 2B. Although FIG. 1shows, for the sake of simplicity, only a single mobile drive unit 20,inventory system 10 may include any appropriate number of mobile driveunits. As a result, mobile drive unit 20 may represent one of severalmobile drive units 20 moving inventory holders 30 in inventory system10.

Inventory holder 30 stores inventory items 40. In particularembodiments, inventory holder 30 includes multiple storage bins witheach storage bin capable of holding inventory items 40. Additionally, inparticular embodiments, inventory items 40 may also hang from hooks orbars within or on inventory holder 30. In general, inventory holder 30may store inventory items 40 in any appropriate manner within inventoryholder 30 and/or on the external surface of inventory holder 30.Inventory holder 30 is capable of being rolled, carried, or otherwisemoved by mobile drive unit 20. The components of a particular embodimentof inventory holder 30 are described in greater detail below withrespect to FIG. 3. Although FIG. 1 shows, for the sake of simplicity,only a single inventory holder 30, inventory system 10 may include anyappropriate number of inventory holders. As a result, inventory holder30 may represent one of several inventory holders 30 storing inventoryitems 40 in inventory system 10.

Inventory items 40 represent any items, material, or animate orinanimate objects suitable for storage, retrieval, delivery, sortation,and/or routing in an automated inventory, warehouse, manufacturing,and/or parts-handling system. As one example, inventory items 40 mayrepresent items of merchandise stored in a warehouse. Mobile drive unit20 may retrieve a specified inventory holder 30 containing particularinventory items 40 associated with a customer order to be packed fordelivery to a customer or other party.

As another example, inventory items 40 may represent luggage stored in aluggage facility of an airport. Mobile drive unit 20 may retrieveinventory holder 30 containing luggage to be transported, tracked,and/or otherwise processed according to particular policies. This mayinclude selecting particular items of luggage for explosives screening,moving items of luggage associated with a flight that has switchedgates, or removing luggage items belonging to passengers who have missedthe flight.

As yet another example, inventory items 40 may represent individualcomponents of a manufacturing kit. More specifically, the components mayrepresent components intended for inclusion in an assembled product,such as computer components for a customized computer system. In such anembodiment, mobile drive unit 20 may retrieve particular componentsidentified by a specification associated with a customer order.

As yet another example, inventory items 40 may represent people. Forexample, in a hospital setting, inventory items 40 may represent bedscontaining particular patients. Thus, inventory system 10 may beconfigured to provide a safe, effective system for moving hospital bedsthat limits the potential for injury to patients and reduces thepossibility of mistakes arising from human error. In general, however,inventory items 40 may be any suitable items appropriate for storage inany appropriate form of inventory holder 30, as described below.

In operation, mobile drive unit 20 is capable of moving between pointswithin a workspace associated with inventory system 10 and, when coupledto inventory holder 30, of transporting inventory holder 30 betweenlocations within the workspace. Mobile drive unit 20 may determine themovement of mobile drive unit 20 autonomously and/or based on commandsreceived by mobile drive unit 20. For example, in particularembodiments, mobile drive unit 20 may receive information thatidentifies destinations for mobile drive unit 20 from a managementdevice of inventory system 10, from an operator of inventory system 10,or any other suitable party or device. Mobile drive unit 20 may receivethe information through a wireless interface, over a wired connection,or using any other suitable components to communicate with an operatoror management device of inventory system 10. In general, however,movement of mobile drive unit 20 may, depending on the configuration ofmobile drive unit 20 and inventory system 10, be controlled, in whole orin part, by mobile drive unit 20, or any appropriate external devices orparties.

For the sake of simplicity, however, the remainder of this descriptionassumes that mobile drive unit 20 wirelessly receives orders, data,instructions, commands, or information structured in any otherappropriate form, referred to here as a “command” or “commands,” from aremote component of inventory system 10. These commands identify aparticular inventory holder 30 to be moved by mobile drive unit 20and/or a current location for that inventory holder 30, and adestination for that inventory holder 30. Mobile drive unit 20 thencontrols operation of motors, wheels, and/or other components of mobiledrive unit 20 to move mobile drive unit 20 and/or inventory holder 30.

In response to receiving such a command, mobile drive unit 20 moves to astorage location identified by the command. Mobile drive unit 20 maythen dock with identified inventory holder 30. Mobile drive unit 20 maydock with inventory holder 30 in any appropriate manner so thatinventory holder 30 is coupled to and/or supported by mobile drive unit20 when mobile drive unit 20 is docked with inventory holder 30. Forexample, in particular embodiments, mobile drive unit 20 docks withinventory holder 30 by positioning itself beneath inventory holder 30and raising a docking head of mobile drive unit 20 until the dockinghead lifts inventory holder 30 off the ground. As a result, in suchembodiments, mobile drive unit 20 may support all or a portion of theweight of inventory holder 30 when mobile drive unit 20 is docked withinventory holder 30. Additionally, in particular embodiments, one ormore components of mobile drive unit 20 may grasp, connect to, interlockwith, or otherwise interact with one or more components of inventoryholder 30 to form a physical coupling between mobile drive unit 20 andinventory holder 30. For example, in particular embodiments, a dockinghead of mobile drive unit 20 may include one or more spines that fitwithin apertures of inventory holder 30 when mobile drive unit 20 dockswith inventory holder 30, allowing mobile drive unit 20 to maneuverinventory holder 30 by applying force to inventory holder 30.

After docking with inventory holder 30, mobile drive unit 20 may moveinventory holder 30 to a second location, such as an inventory station,where appropriate inventory items 40 may be selected from inventoryholder 30 and packed for shipping or where inventory items 40 may beadded to inventory holder 30 to replenish the supply of inventory items40 available in inventory system 10. In a particular embodiment, mobiledrive unit 20 may provide sufficient power to propel both mobile driveunit 20 and inventory holder 30. In alternative embodiments, inventoryholder 30 may provide additional power, such as through the operation ofmotorized wheels on inventory holder 30, to assist mobile drive unit 20in propelling inventory holder 30 to the second location.

Depending on the configuration and characteristics of mobile drive unit20 and inventory system 10, mobile drive unit 20 may move inventoryholder 30 using a variety of appropriate methods. In a particularembodiment, mobile drive unit 20 is capable of moving inventory holder30 along a two-dimensional grid, combining movement along straight-linesegments with ninety-degree rotations and arcing paths to transportinventory holder 30 from the first location to the second location.Additionally, while moving, mobile drive unit 20 may use fixed objectslocated in the workspace as reference points to assist in navigation.For example, in particular embodiments, inventory system 10 includesmultiple fiducial marks 50. Mobile drive unit 20 may be configured todetect fiducial marks 50 and to determine the location of mobile driveunit 20 and/or measure its movement based on the detection of fiducialmarks 50.

After mobile drive unit 20 arrives at the second location, mobile driveunit 20 may perform appropriate operations to facilitate access toinventory items 40 stored in inventory holder 30. For example, mobiledrive unit 20 may rotate inventory holder 30 to present a particularface of inventory holder 30 to an operator of inventory system 10 orother suitable party, such as a packer selecting inventory items 40 frominventory holder 40. Mobile drive unit 20 may also undock from inventoryholder 30. Alternatively, instead of undocking at the second location,mobile drive unit 20 may transport inventory holder 30 back to the firstlocation or to a third location after any appropriate actions have beentaken involving inventory items 40. For example, after a packer hasremoved particular inventory items 40 from inventory holder 30, mobiledrive unit 20 may return inventory holder 30 to its original storagelocation, a new storage location, or another inventory station. Mobiledrive unit 20 may then undock from inventory holder 30 at this newlocation.

Because inventory system 10, in particular embodiments, is at leastpartially automated, it may be desirable for the location of inventoryholder 30 to be precisely controlled so that mobile drive units 20 canproperly locate and interact with inventory holders 30. As used in thisdescription and the claims that follow, unless otherwise indicated the“location” of an inventory holder 30 or mobile drive unit 20 mayrepresent an absolute or relative measurement of the location, position,and/or orientation of the relevant device. As a result, before undockingfrom inventory holder 30, mobile drive unit 20 may attempt to alignitself with a reference point within the workspace so that inventoryholder 30 will be properly situated for subsequent transportation and/orto prevent collision with other devices moving within the workspace. Forexample, mobile drive unit 20 may attempt to align itself with aparticular fiducial mark 50 before undocking from inventory holder 30.

While mobile drive unit 20 is transporting inventory holder 30, however,the location of inventory holder 30 relative to mobile drive unit 20 mayshift slightly. This shifting may occur as the result of slippagebetween docking surfaces of mobile drive unit 20 and inventory holder30, vibration during transport, or any other incident that may alter thelocation of inventory holder 30 relative to mobile drive unit 20. As aresult of this slippage, mobile drive unit's attempt to align itselfwith an appropriate reference point before undocking may not besufficient to ensure that inventory holder is properly aligned with thatreference point after undocking

Consequently, in particular embodiments, mobile drive unit 20 may beconfigured to perform certain operations, as described in greater detailbelow, to detect and correct misalignments of an inventory holder 30after that inventory holder 30 has been moved within the workspace. Morespecifically, in particular embodiments, mobile drive unit 20 maydetermine the difference between the location of a particular inventoryholder 30 and an anticipated, intended, and/or requested location of theinventory holder 30. Mobile drive unit 20 may then take certain actionsto reduce or eliminate this deviation, immediately following undockingfrom inventory holder 30, prior to a subsequent docking with inventoryholder 30, or at any other appropriate time during the course ofoperation. FIGS. 4A-4H illustrate operation of a particular embodimentof mobile drive unit 20 in correcting a misalignment of inventory holder30. Additionally, FIGS. 5 and 6 are flowcharts describing the operationof particular embodiments of mobile drive unit 20 in correctingmisalignments in inventory holder 30.

The implementation of such correction techniques may, in particularembodiments, allow greater precision in the placement of inventoryholders 30 and facilitate a greater degree of automation in inventorysystem 10. Additionally, in particular embodiments, implementation ofsuch techniques may also reduce operational errors in inventory system10, permit the use of a smaller workspace, and/or prevent improperlypositioned inventory holders 30 from being damaged by mobile drive units20 and other components moving in the workspace of inventory system 10.As a result, particular embodiments of mobile drive unit 20 may providemultiple benefits. Nonetheless, although particular embodiments ofmobile drive unit 20 may provide such benefits, alternative embodimentsmay provide some, none, or all of these benefits.

FIGS. 2A and 2B include a front and side view of a particular embodimentof mobile drive unit 20. Mobile drive unit 20 includes a docking head110, a drive module 120, a docking actuator 130, and a control module160. Additionally, mobile drive unit 20 may include one or more sensorsconfigured to detect or determine the location of mobile drive unit 20,inventory holder 30, and/or other appropriate elements of inventorysystem 10. In the illustrated embodiment, mobile drive unit 20 includesa position sensor 140 and a holder sensor 150.

Docking head 110, in particular embodiments of mobile drive unit 20,couples mobile drive unit 20 to inventory holder 30 and/or supportsinventory holder 30 when mobile drive unit 20 is docked to inventoryholder 30. Docking head 110 may additionally allow mobile drive unit 20to maneuver inventory holder 30, such as by lifting inventory holder 30,propelling inventory holder 30, rotating inventory holder 30, and/ormoving inventory holder 30 in any other appropriate manner. Docking head110 may also include any appropriate combination of components, such asribs, spikes, and/or corrugations, to facilitate such manipulation ofinventory holder 30. For example, in particular embodiments, dockinghead 110 may include a high-friction portion that abuts a portion ofinventory holder 30 while mobile drive unit 20 is docked to inventoryholder 30. In such embodiments, frictional forces created between thehigh-friction portion of docking head 110 and a surface of inventoryholder 30 may induce translational and rotational movement in inventoryholder 30 when docking head 110 moves and rotates, respectively. As aresult, mobile drive unit 20 may be able to manipulate inventory holder30 by moving or rotating docking head 110, either independently or as apart of the movement of mobile drive unit 20 as a whole.

Drive module 120 propels mobile drive unit 20 and, when mobile driveunit 20 and inventory holder 30 are docked, inventory holder 30. Drivemodule 120 may represent any appropriate collection of componentsoperable to propel drive module 120. For example, in the illustratedembodiment, drive module 120 includes motorized axle 122, a pair ofmotorized wheels 124, and a pair of stabilizing wheels 126. Onemotorized wheel 124 is located at each end of motorized axle 122, andone stabilizing wheel 126 is positioned at each end of mobile drive unit20.

Docking actuator 130 moves docking head 110 towards inventory holder 30to facilitate docking of mobile drive unit 20 and inventory holder 30.Docking actuator 130 may also be capable of adjusting the position ororientation of docking head 110 in other suitable manners to facilitatedocking Docking actuator 130 may include any appropriate components,based on the configuration of mobile drive unit 20 and inventory holder30, for moving docking head 110 or otherwise adjusting the position ororientation of docking head 110. For example, in the illustratedembodiment, docking actuator 130 includes a motorized shaft (not shown)attached to the center of docking head 110. The motorized shaft isoperable to lift docking head 110 as appropriate for docking withinventory holder 30.

Drive module 120 may be configured to propel mobile drive unit 20 in anyappropriate manner. For example, in the illustrated embodiment,motorized wheels 124 are operable to rotate in a first direction topropel mobile drive unit 20 in a forward direction. Motorized wheels 124are also operable to rotate in a second direction to propel mobile driveunit 20 in a backward direction. In the illustrated embodiment, drivemodule 120 is also configured to rotate mobile drive unit 20 by rotatingmotorized wheels 124 in different directions from one another or byrotating motorized wheels 124 at different speed from one another.

Position sensor 140 represents one or more sensors, detectors, or othercomponents suitable for determining the location of mobile drive unit 20in any appropriate manner. For example, in particular embodiments, theworkspace associated with inventory system 10 includes a number offiducial marks that mark points on a two-dimensional grid that coversall or a portion of the workspace. In such embodiments, position sensor140 may include a camera and suitable image- and/or video-processingcomponents, such as an appropriately-programmed digital signalprocessor, to allow position sensor 140 to detect fiducial marks withinthe camera's field of view. Control module 160 may store locationinformation that position sensor 140 updates as position sensor 140detects fiducial marks. As a result, position sensor 140 may utilizefiducial marks to maintain an accurate indication of the location mobiledrive unit 20 and to aid in navigation when moving within the workspace.

Holder sensor 150 represents one or more sensors, detectors, or othercomponents suitable for detecting inventory holder 30 and/ordetermining, in any appropriate manner, the location of inventory holder30, as an absolute location or as a position relative to mobile driveunit 20. Holder sensor 150 may be capable of detecting a particularportion of inventory holder 30 or inventory holder 30 as a whole. Forexample, in particular embodiments, each inventory holder 30 ininventory system 10 includes a holder identifier that marks apredetermined location on that inventory holder 30. In certain examplesof such embodiments, mobile drive unit 20 and inventory holder 30 mayalso be configured so that, when mobile drive unit 20 moves beneathinventory holder 30, mobile drive unit 20 is able to detect the holderidentifier directly. In such embodiments, holder sensor 150 may includea camera and suitable image- and/or video-processing components, such asan appropriately-programmed digital signal processor, to allow holdersensor 150 to detect the holder identifier of a particular inventoryholder 30 and to calculate the location of the inventory holder 30 basedon this detection. As a result, mobile drive unit 20 may be configuredto utilize these holder identifiers as reference points to aid innavigation when docking with or undocking from inventory holders 30.

Control module 160 monitors and/or controls operation of drive module120 and docking actuator 130. Control module 160 may also receiveinformation from sensors such as position sensor 140 and holder sensor150 and adjust the operation of drive module 120, docking actuator 130,and/or other components of mobile drive unit 20 based on thisinformation. Additionally, in particular embodiments, mobile drive unit20 may be configured to communicate with a management device ofinventory system 10 and control module 160 may receive commandstransmitted to mobile drive unit 20 and communicate information back tothe management device utilizing appropriate communication components ofmobile drive unit 20. Control module 160 may include any appropriatehardware and/or software suitable to provide the describedfunctionality. In particular embodiments, control module 160 includes ageneral-purpose microprocessor programmed to provide the describedfunctionality. Additionally, control module 160 may include all orportions of docking actuator 120, drive module 130, position sensor 140,and/or holder sensor 150, and/or share components with any of theseelements of mobile drive unit 20.

FIG. 3 illustrates inventory holder 30 in accordance with a particularembodiment. FIG. 3 illustrates the structure and contents of one side ofinventory holder 30. In a particular embodiment, inventory holder 30 maycomprise any number of faces with similar or different structure. Asillustrated, inventory holder 30 includes a frame 310, a plurality oflegs 328, and docking surface 350.

Frame 310 holds inventory items 40. Frame 310 provides storage space forstoring inventory items 40 external or internal to frame 310. Thestorage space provided by frame 310 may be divided into a plurality ofinventory bins 320, each capable of holding inventory items 40.Inventory bins 320 may include any appropriate storage elements, such asbins, compartments, or hooks.

In a particular embodiment, frame 310 is composed of a plurality oftrays 322 stacked upon one another and attached to or stacked on a base318. In such an embodiment, inventory bins 320 may be formed by aplurality of adjustable dividers 324 that may be moved to resize one ormore inventory bins 320. In alternative embodiments, frame 310 mayrepresent a single inventory bin 320 that includes a single tray 322 andno adjustable dividers 324. Additionally, in particular embodiments,frame 310 may represent a load-bearing surface mounted on mobilityelement 330. Inventory items 40 may be stored on such an inventoryholder 30 by being placed on frame 310. In general, frame 310 mayinclude storage internal and/or external storage space divided into anyappropriate number of inventory bins 320 in any appropriate manner.

Additionally, in a particular embodiment, frame 310 may include aplurality of device openings 326 that allow mobile drive unit 20 toposition docking head 110 adjacent docking surface 350. The size, shape,and placement of device openings 326 may be determined based on thesize, the shape, and other characteristics of the particular embodimentof mobile drive unit 20 and/or inventory holder 30 utilized by inventorysystem 10. For example, in the illustrated embodiment, frame 310includes four legs 328 that form device openings 326 and allow mobiledrive unit 20 to position mobile drive unit 20 under frame 310 andadjacent to docking surface 350. The length of legs 328 may bedetermined based on a height of mobile drive unit 20.

Docking surface 350 comprises a portion of inventory holder 30 thatcouples to, abuts, and/or rests upon a portion of docking head 110, whenmobile drive unit 20 is docked to inventory holder 30. Additionally,docking surface 350 supports a portion or all of the weight of inventoryholder 30 while inventory holder 30 is docked with mobile drive unit 20.The composition, shape, and/or texture of docking surface 350 may bedesigned to facilitate maneuvering of inventory holder 30 by mobiledrive unit 20. For example, as noted above, in particular embodiments,docking surface 350 may comprise a high-friction portion. When mobiledrive unit 20 and inventory holder 30 are docked, frictional forcesinduced between docking head 110 and this high-friction portion mayallow mobile drive unit 20 to maneuver inventory holder 30.Additionally, in particular embodiments, docking surface 350 may includeappropriate components suitable to receive a portion of docking head110, couple inventory holder 30 to mobile drive unit 20, and/orfacilitate control of inventory holder 30 by mobile drive unit 20.

Holder identifier 360 marks a predetermined portion of inventory holder30 and mobile drive unit 20 may use holder identifier 360 to align withinventory holder 30 during docking and/or to determine the location ofinventory holder 30. More specifically, in particular embodiments,mobile drive unit 20 may be equipped with components, such as holdersensor 150, that can detect holder identifier 360 and determine itslocation relative to mobile drive unit 20. As a result, mobile driveunit 20 may be able to determine the location of inventory holder 30 asa whole. For example, in particular embodiments, holder identifier 360may represent a reflective marker that is positioned at a predeterminedlocation on inventory holder 30 and that holder sensor 150 can opticallydetect using an appropriately-configured camera.

Additionally, in particular embodiments, inventory system 10 includesmultiple inventory holders 30 and, in such embodiments, holderidentifier 360 may uniquely identify the particular inventory holder 30to which holder identifier is attached. As one example, in particularembodiments, holder identifier 360 may include a radio-frequency (RF)emitter that transmits an RF signal uniquely identifying inventoryholder 30. As mobile drive unit 20 approaches a particular inventoryholder 30, mobile drive unit 20 may be configured to detect the RFsignal and determine which inventory holder 30 mobile drive unit 20 isapproaching based on the detected RF signal. As another example, inparticular embodiments, holder identifier 360 represents marker bearinga bar-code, and mobile drive unit 20 may be configured to read thisbar-code while passing under inventory holder 30 and determine whichinventory holder 30 mobile drive unit 20 is passing beneath based on thecontents of the bar-code. Moreover, in such embodiments, the bar-codemay represent a two-dimensional bar-code to allow more information to bestored in the bar-code and, thus, a greater number of inventory holders30 may be utilized within inventory system 10.

FIGS. 4A-4H illustrate operation of particular embodiments of mobiledrive unit 20 and inventory holder 30 during docking, movement, andundocking For purposes of example, FIGS. 4A-4H illustrate particularembodiments of mobile drive unit 20 and inventory holder 30 configuredto couple and uncouple while mobile drive unit 20 is positioned beneathinventory holder 30. As noted above, however, alternative embodiments ofmobile drive unit 20 and inventory holder 30 may be configured to couplewhile mobile drive unit 20 is positioned in any appropriate mannerrelative to inventory holder 30. Additionally, although FIGS. 4A-4Hillustrate operation of a particular embodiment of mobile drive unit 20configured to detect its location and the location of inventory holder30 using optical sensors, alternative embodiments of mobile drive unit20 may be configured to determine the relevant locations using otherappropriate forms of sensors, as described above.

FIG. 4A illustrates mobile drive unit 20 and inventory holder 30 priorto docking As noted above with respect to FIG. 1, mobile drive unit 20may receive a command that identifies a location for a particularinventory holder 30. Mobile drive unit 20 may then move to the locationspecified in the command. Additionally, mobile drive unit 20 may utilizeposition sensor 140 to determine the location of mobile drive unit 20 toassist in navigating to the location of inventory holder 30.

In particular, FIG. 4A shows mobile drive unit 20 and inventory holder30 as mobile drive unit 20 approaches the storage location identified bythe received command. In the illustrated embodiment, the reference pointis marked by fiducial mark 50 a which comprises a surface operable toreflect light and which, as a result, can be detected by particularembodiments of position sensor 140 when mobile drive unit 20 ispositioned over or approximately over fiducial mark 50 a. As notedabove, the illustrated embodiment of mobile drive unit 20 utilizesoptical sensors, including a camera and appropriate image- and/or videoprocessing components, to detect fiducial marks 50.

FIG. 4B illustrates mobile drive unit 20 and inventory holder 30 oncemobile drive unit 20 reaches fiducial mark 50 a. Because, in theillustrated example, fiducial mark 50 a marks the location of thereference point to which mobile drive unit 20 is destined, mobile driveunit 20 begins the docking process once mobile drive unit 20 reachesfiducial mark 50 a. In the illustrated example, mobile drive unit 20 isconfigured to dock with inventory holder 30 from a position beneathinventory holder 30 and, as a result, inventory holder 30 is stored sothat docking surface 350 is located directly above fiducial mark 50 a.

Although in the illustrated example, inventory holder 30, at this point,is assumed to be aligned with or within some tolerance of being alignedwith fiducial mark 50 a, particular embodiments of mobile drive unit 20may be configured to additionally determine whether inventory holder 30is aligned with fiducial mark 50 a and, if not, to execute anyappropriate additional steps to facilitate docking despite themisalignment of inventory holder 30. For example, in particularembodiments, mobile drive unit 20 may determine whether inventory holder30 is appropriately aligned with fiducial mark 50 a, such as by aligningitself with fiducial mark 50 a and determining a difference between thelocation of mobile drive unit 20 and the location of inventory holder 30or an appropriate portion of inventory holder 30. If the differencebetween the location of mobile drive unit 20 and the location ofinventory holder 30 is greater than some predetermined tolerance, mobiledrive unit 20 may then adjust its position based on the detectedlocation of inventory holder 30 to facilitate docking

FIG. 4C illustrates operation of mobile drive unit 20 in docking withinventory holder 30. After positioning itself over fiducial mark 50 a,mobile drive unit 20 begins the docking process. In the illustratedexample, the docking process includes mobile drive unit 20 raisingdocking head 110 towards docking surface 350, as indicated by arrow 510.Additionally, in the illustrated example, mobile drive unit 20 andinventory holder 30 are configured so that mobile drive unit 20 liftsinventory holder 30 off the ground when mobile drive unit 20 docks withinventory holder 30 and, as a result, mobile drive unit 20 supports theweight of inventory holder 30 while mobile drive unit 20 is docked toinventory holder 30.

FIG. 4D illustrates operation of mobile drive unit 20 after docking withinventory holder 30. Mobile drive unit 20 is capable of inducingtranslational and/or rotational movement in inventory holder 30 whilemobile drive unit 20 is docked with inventory holder 30. For example, inthe illustrated embodiment, inventory holder 30 is supported by mobiledrive unit 20 while the two components are docked and mobile drive unit20 is capable of inducing translational and/or rotational movement ininventory holder 30 by moving or rotating itself or some sub-componentof itself, such as docking head 110. As a result, while mobile driveunit 20 and inventory holder 30 are docked mobile drive unit 20 may moveinventory holder 30 to a requested destination based on commandsreceived by mobile drive unit 20, as suggested by arrow 520.

More specifically, after docking with inventory holder 30, mobile driveunit 20 may then move inventory holder 30 to the destination identifiedin the command received by mobile drive unit 20, propelling and/orrotating inventory holder 30 as appropriate. In particular embodiments,mobile drive unit 20 may monitor and update a calculation of its ownlocation as it moves. For example, in particular embodiments, mobiledrive unit 20 detects one or more fiducial marks 50 along the path tothe requested destination and updates its location based on thedetection of these fiducial marks 50. As a result, mobile drive unit 20may be able to maintain an up-to-date approximation of its locationwhile moving.

Once mobile drive unit 20 and inventory holder 30 arrive at thedestination, mobile drive unit 20 may additionally rotate inventoryholder 30 to present a particular face of inventory holder 30 to apacker or otherwise maneuver inventory holder 30 to allow access toinventory items 40 stored by inventory holder 30. Mobile drive unit 20may then undock from inventory holder 30, as described below, or moveinventory holder to another destination. For example, mobile drive unit20 may move inventory holder 30 to a packing station where a packer canselect appropriate inventory items 40 from inventory holder 30. Mobiledrive unit 20 may then return inventory holder 30 to its originallocation or another location appropriate for undocking, such as a newstorage location reserved for inventory holder 30.

FIG. 4E illustrates mobile drive unit 20 and inventory holder 30 whenthe two components arrive at an appropriate point for undocking As notedabove, this may represent a final destination specified by the originalcommand, the original storage location for inventory holder 30, or anyother point within the workspace. At or near the destination, mobiledrive unit 20 may detect another fiducial mark 50, fiducial mark 50 b,associated with the undocking location. Mobile drive unit 20 determinesits location based on fiducial mark 50 b and, as a result, determinesthat it has reached the undocking location.

After determining that it has reached the undocking location, mobiledrive unit 20 initiates an appropriate undocking process based on theconfiguration and characteristics of mobile drive unit 20 and inventoryholder 30. As part of this process, mobile drive unit 20 may alignmobile drive unit 20 with one or more fiducial markers 50 or otherappropriate reference points on the workspace before undocking Aligningwith these reference points may allow mobile drive unit 20 to aligninventory holder 30 to a grid associated with the workspace and, thus,to ensure that mobile drive unit 20 or other components of inventorysystem 10 can move within the workspace without interfering withinventory holder 30. After aligning mobile drive unit 20 based on thereference points, mobile drive unit 20 may undock from inventory holder30. Mobile drive unit 20 may then execute any appropriate additionalsteps in undocking from inventory holder 30. For example, in theillustrated embodiment, mobile drive unit 20 supports inventory holder30 on docking head 110 while mobile drive unit 20 and inventory holder30 are docked. As a result, in this embodiment, mobile drive unit 20undocks from inventory holder 30, in part, by lowering docking head 110.As mobile drive unit 20 lowers docking head 110, the weight of inventoryholder 30 will be shifted from mobile drive unit 20 to legs 328.

Additionally, the location of inventory holder 30 relative to mobiledrive unit 20 may shift during transit. As a result, the location ofinventory holder 30 may not be properly aligned with fiducial marker 50b after undocking despite any steps taken by mobile drive unit 20 toalign itself to fiducial marker 50 b. Moreover, in particularembodiments, mobile drive units 20 may use the expected location ofinventory holders to navigate within the workspace of inventory system10. As a result, if inventory holder 30 is left at the requesteddestination without being properly aligned with fiducial marker 50 b,other mobile drive units 20 may interfere with inventory holder 30,other mobile drive units 20 may subsequently have difficulty dockingwith inventory holder 30, and/or other related problems may arise.Consequently, mobile drive unit 20 may attempt to verify that inventoryholder 30 is properly aligned with fiducial mark 50 b and, if not, takeappropriate steps to align inventory holder 30 with fiducial mark 50 b.

For example, in particular embodiments, mobile drive unit 20 may, afterundocking from inventory holder 30 determine a location of inventoryholder 30. Mobile drive unit 20 may utilize holder sensor 150 to detectthe location of inventory holder 30. Based on the information detectedby holder sensor 150, mobile drive unit 20 may determine the differencebetween the location of mobile drive unit 20 and the location ofinventory holder 30. If the difference is greater than somepredetermined tolerance, mobile drive unit 20 may attempt to moveinventory holder 30 to more closely align inventory holder 30 with thecurrent location of mobile drive unit 20 and/or fiducial mark 50 b, asshown in FIGS. 4F-4H.

FIGS. 4F-4H illustrate operation of a particular embodiment of mobiledrive unit 20 after determining that the location of inventory holder 30differs from the location of mobile drive unit 20 and/or fiducial mark50 b by more than the predetermined tolerance. More specifically, FIG.4F illustrates mobile drive unit 20 as mobile drive unit 20 positionsitself under inventory holder 30. In particular, after determining thatthe location of inventory holder 30 differs from the current location ofmobile drive unit 20 and/or that of fiducial mark 50 b by more than thepredetermined tolerance, mobile drive unit 20, in the illustratedembodiment, moves to a new location based on the detected location ofinventory holder 30, as shown by arrow 530. In the illustratedembodiment, mobile drive unit 20 repositions itself so that holdersensor 150 is located under holder identifier 360. Mobile drive unit 20then docks with inventory holder 30 again in a similar manner to thatshown in FIG. 4C.

FIG. 4G illustrates the operation of a particular embodiment of mobiledrive unit 20 after docking with inventory holder 30 a second time tocorrect the location of inventory holder 30. After docking withinventory holder 30 again, mobile drive unit 20 moves back towards therequested destination, as suggested by arrow 540. Mobile drive unit 20may continue moving towards the requested destination until mobile driveunit 20 detects fiducial mark 50 b again.

FIG. 4H illustrates the operation of a particular embodiment of mobiledrive unit 20 after reaching the requested destination a second time.After aligning with fiducial mark 50 b again, mobile drive unit 20undocks from inventory holder 30. In the illustrated embodiment, mobiledrive unit 20 undocks from inventory holder 30 by lowering docking head110 as suggested by arrow 550. Because, in particular embodiments, themovement to correctly align inventory holder 30 with the requesteddestination will often be significantly smaller than the movement fromthe original location to the requested destination, the chances of anyslippage, jostling, or other error-inducing incidents will be muchsmaller. Consequently, this correction technique may substantiallyimprove the precision with which inventory holder 30 can be stored.

Mobile drive unit 20 may then move away from inventory holder 30 andbegin responding to other commands received by mobile drive unit 20.Alternatively, mobile drive unit 20 may repeat the determination of thelocation of inventory holder 30 and again attempt to align inventoryholder 30 to fiducial mark 50 b. Moreover, mobile drive unit 20 mayperform these steps until completing a predetermined number ofiterations or indefinitely until inventory holder 30 is finally alignedwith fiducial mark 50 b. Repeating this process may improve theprecision of the inventory holder's storage beyond the improvementproduced by the initial correction.

In alternative embodiments, mobile drive unit 20 may use informationfrom sensor 140 and sensor 150 to calculate the relative offset ofinventory holder 30 from desired location 50 b and then make correctivemoves prior to undocking that results in inventory holder 30 beingpositioned within tolerance to fiducial 50 b. This process reduces theamount of time required to accurately place the inventory holder 30 onthe grid. Additionally, mobile drive unit 20 may perform this differencecalculation at any point along a path to or from storage destinationsand make a determination that it should undock and redock to improve thelocation of the inventory holder 30 relative to the drive unit 20. Anexample of this operation is discussed below with respect to FIG. 7.

Thus, in particular embodiments of inventory system 10, mobile driveunit 20 may be capable of using certain techniques to correct thelocation of inventory holder 30 after transporting inventory holder 30between points within the workspace of inventory system 10. This mayreduce the occurrence of operational errors in inventory system 10.Additionally, in particular embodiments, implementation of suchtechniques may also allow for greater automation of the operation ofinventory system 10, permit the use of a smaller workspace, and/orprevent improperly positioned inventory holders 30 from interfering withthe normal operation of the inventory system 10. As a result, particularembodiments of mobile drive unit 20 may provide multiple benefits.Nonetheless, although particular embodiments of mobile drive unit 20 mayprovide such benefits, alternative embodiments may provide some, none,or all of these benefits.

Additionally, although FIGS. 4A-4H focus, for the sake of illustration,on an embodiment of mobile drive unit 20 that detects and correctmisalignments of inventory holder 30 immediately following undocking, inalternative embodiments, mobile drive unit 20 may be configured todetect a misalignment at other appropriate times after mobile drive unit20 has moved inventory holder 30. Moreover, the techniques utilized bymobile drive unit 20 to detect and correct the misalignment may varydepending on when mobile drive unit 20 detects and/or corrects themisalignment. As discussed below, FIG. 5 is a flowchart detailing atechnique utilized by particular embodiments of mobile drive unit 20 todetect and correct misalignments of inventory holder 30 immediatelyafter undocking, similar to the embodiment and techniques illustrated inFIGS. 4A-4H. Furthermore, FIG. 6 is a flowchart detailing a techniqueutilized by particular embodiments of mobile drive unit 20 to detect andcorrect misalignments of inventory holder 30 before subsequent dockingswith that inventory holder 30. In general, however, mobile drive unit 20may be configured to detect and/or correct the misalignment of inventoryholder 30 at any appropriate time using any suitable techniques.

FIG. 5 is a flowchart illustrating operation of a particular embodimentof mobile drive unit 20 during the docking and undocking processillustrated by FIGS. 4A-4H. Some of the steps illustrated in FIG. 5 maybe combined, modified, or deleted where appropriate, and additionalsteps may also be added to the flowchart. Additionally, steps may beperformed in any suitable order without departing from the scope of theinvention.

Operation begins at step 600 with mobile drive unit 20 receiving acommand identifying an inventory holder 30 to be moved or a location foran inventory holder 30 to be moved. In response to receiving thecommand, mobile drive unit 20 moves to or near the starting location forthe identified inventory holder 30 at step 602. In particularembodiments, mobile drive unit 20 may position mobile drive unit 20under the identified inventory holder 30, at step 604, or perform anyother adjustments to align mobile drive unit 20 with inventory holder30, prepare mobile drive unit 20 or inventory holder 30 for docking,and/or otherwise facilitate docking and transport.

Mobile drive unit 20 may then initiate docking at step 606. Inparticular embodiments, mobile drive unit 20 initiates docking byraising docking head 110. When docking head 110 contacts docking surface350 of inventory holder 30, mobile drive unit 20 begins liftinginventory holder 30 off the ground. As a result, in such embodiments,mobile drive unit 20 supports inventory holder 30 once mobile drive unit20 is docked with inventory holder 30.

At step 608, mobile drive unit 20 moves inventory holder 30 towards adestination requested by the received command. As noted above, whilemoving, mobile drive unit 20 may continually monitor and/or periodicallymake an updated determination of the location of mobile drive unit 20and/or the relative positions of mobile drive unit 20 and inventoryholder 30. In particular embodiments, mobile drive unit 20 may monitoror determine the location of mobile drive unit 20 by detecting fixedobjects, such as fiducial markers 50, as mobile drive unit 20 moves.

At step 610, mobile drive unit 20 determines that mobile drive unit 20has reached the requested destination. In particular embodiments, mobiledrive unit 20 may determine that mobile drive unit 20 has reached therequested determination by detecting a fixed object, such as a fiducialmarker 50 associated with the requested destination. Upon reaching therequested destination, mobile drive unit 20 undocks from inventoryholder 30 at step 612. In particular embodiments, mobile drive unit 20may undock by lowering a docking head 110 that is supporting inventoryholder 30.

After undocking, mobile drive unit 20, mobile drive unit 20 maydetermine the location of inventory holder 30 at step 614.Alternatively, in certain embodiments, mobile drive unit 20 maydetermine the location of inventory holder 30 prior to undocking Ingeneral, however, mobile drive unit 20 may include a holder sensor 150that is capable of detecting some portion or feature of inventoryholders 30, such as a holder identifier 360, that holder sensor 150 canuse to determine the location of the relevant inventory holder 30relative to mobile drive unit 20. Mobile drive unit 20 may thencalculate, at step 616, a difference between the location of inventoryholder 30 and the requested destination. At step 618, mobile drive unit20 may determine whether the difference is greater than a predeterminedtolerance. If the difference is not greater than the predeterminedtolerance, mobile drive unit 20 may then move away from inventory holder30, at step 620, and proceed with responding to other commands. Theoperation of mobile drive unit 20, with respect to moving this inventoryholder 30, may then end, as shown in FIG. 5.

If instead, the difference is greater than the predetermined tolerance,mobile drive unit 20 may move mobile drive unit 20 to another pointbased on the determined location of inventory holder 30 and/or thedifference between the determined location of inventory holder 30 andthe requested destination. For example, in particular embodiments, suchas the one illustrated in FIGS. 4A-4H, mobile drive unit 20 may move tothe detected location of inventory holder 30 at step 622. Mobile driveunit 20 then docks with inventory holder 30 again at step 624.

After docking with inventory holder 30 again, mobile drive unit 20 movesinventory holder 30 towards the requested destination at step 626. Whenmobile drive unit 20 reaches the requested destination, mobile driveunit 20 undocks from inventory holder 30 again at step 628. Mobile driveunit 20 may then move away from inventory holder 30, at step 630, andproceed with responding to other commands. The operation of mobile driveunit 20, with respect to moving this inventory holder 30, may then end,as shown in FIG. 5. Alternatively, in particular embodiments, mobiledrive unit 20 may determine the location of inventory holder 30 againand repeat any of the above steps again as appropriate based on theconfiguration of mobile drive unit 20.

Additionally, in alternative embodiments, mobile drive unit 20 maydetect and correct misalignments in inventory holder 30 before undockingfrom inventory holder 30 the first time. This may allow mobile driveunit 20 to reduce the time and/or power expended in correctingmisalignments. An example of such a process is illustrated below withrespect to FIG. 7.

FIG. 6 is a flowchart illustrating operation of an alternativeembodiment of mobile drive unit 20 in detecting and correcting amisalignment of inventory holder 30 subsequent to inventory holder 30being moved within the workspace of inventory system 10. Any of thesteps illustrated in FIG. 6 may be combined, modified, or deleted whereappropriate, and additional steps may also be added to the flowchart.Additionally, steps may be performed in any suitable order withoutdeparting from the scope of the invention.

In the example, described by the flowchart in FIG. 6, mobile drive unit20 begins operation undocked from any inventory holder 30. Operationbegins at step 700 with mobile drive unit 20 receiving a commandidentifying an inventory holder 30 to be moved or a location for aninventory holder 30 to be moved and a desired destination for thatinventory holder 30. In response to receiving the command, mobile driveunit 20 moves to or near the starting location for the identifiedinventory holder 30 at step 702. In particular embodiments, mobile driveunit 20 may position mobile drive unit 20 under the identified inventoryholder 30, at step 704, or perform any other adjustments to align mobiledrive unit 20 with inventory holder 30, prepare mobile drive unit 20 orinventory holder 30 for docking, and/or otherwise facilitate docking andtransport.

Additionally, in the described embodiment, mobile drive unit 20 maydetermine the location of inventory holder 30 at step 706. At step 708,mobile drive unit 20 may determine a difference between the location ofinventory holder 30 and the designated location for that inventoryholder 30. For example, the command received by mobile drive unit 20 mayidentify a location at which this same mobile drive unit 20 or anothermobile drive unit 20 previously stored the relevant inventory holder 30.Mobile drive unit 20 may move to the designated location, determine theactual location of inventory holder 30 once at or near the designatedlocation, and calculate a difference between the actual location and thedesignated location.

If the difference between the actual location of inventory holder 30 andthe designated location of inventory holder 30 is greater than somepredetermined tolerance, in particular embodiments, mobile drive unit 20may take appropriate remedial action to correct the misalignment. Forexample, in the described embodiment, mobile drive unit 20 calculates acorrection value based on the difference between the designated locationand the actual location at step 710.

Additionally, in the described embodiment, mobile drive unit 20determines an adjusted destination using the correction value at step712. For example, if mobile drive unit 20 determines upon arriving atthe designated location that the relevant inventory holder 30 is aquarter inch east of the designated location for that inventory holder30, mobile drive unit 20 may determine an adjusted destination that is aquarter inch west of the destination location specified by the command.

At step 714, mobile drive unit 20 may dock with inventory holder 30.Mobile drive unit 20 may then move mobile drive unit 20 and inventoryholder 30 to the adjusted destination at step 716. At step 718, mobiledrive unit 20 may undock from inventory holder 30 at the adjusteddestination. In particular embodiments, mobile drive unit 20 may takeadditional steps to detect and/or correct any remaining or subsequentlyoccurring misalignment of inventory holder 30 after arriving at theadjusted destination and/or after undocking to further reduce thepotential for misalignments. For example, in particular embodiments,mobile drive unit 20 may, after undocking, attempt to detect and correctany misalignments using techniques similar to those described above inFIG. 5. Mobile drive unit 20 may then move away from inventory holder30, at step 740, and proceed with responding to other commands. Theoperation of mobile drive unit 20, with respect to moving this inventoryholder 30, may then end, as shown in FIG. 6.

As noted above, the steps illustrated in FIG. 6 may be performed in anyappropriate order. For example, instead of determining the location ofinventory holder 30 prior to docking, mobile drive unit 20 mayalternatively determine the location of inventory holder 30 relative tomobile drive unit 20 after docking with inventory holder 30 and while intransit to the destination location. Mobile drive unit 20 may then usethis relative location of inventory holder 30 to calculate theadjustment factor and to determine the adjusted destination. An exampleof such a process is illustrated below with respect to FIG. 8.

FIG. 7 is a flowchart illustrating another example technique that may beutilized by particular embodiments of mobile drive units 20 to detectand correct misalignments when transporting inventory holders 30. Inparticular, FIG. 7 illustrates a technique in which, after moving aninventory holder 30 to a requested destination, mobile drive unit 20 maydetect a misalignment in the relevant inventory holder 30 prior toundocking from that inventory holder 30. Some of the steps illustratedin FIG. 7 may be combined, modified, or deleted where appropriate, andadditional steps may also be added to the flowchart. Furthermore, stepsmay be performed in any suitable order without departing from the scopeof the invention.

In this embodiment, operation begins at step 800 with mobile drive unit20 receiving a command identifying an inventory holder 30 to be moved ora location for an inventory holder 30 to be moved. In response toreceiving the command, mobile drive unit 20 moves to or near thestarting location for the identified inventory holder 30 at step 802. Inparticular embodiments, mobile drive unit 20 may position mobile driveunit 20 under the identified inventory holder 30, at step 804, orperform any other adjustments to align mobile drive unit 20 withinventory holder 30, prepare mobile drive unit 20 or inventory holder 30for docking, and/or otherwise facilitate docking and transport.

Mobile drive unit 20 may then initiate docking at step 806. Inparticular embodiments, mobile drive unit 20 initiates docking byraising docking head 110. When docking head 110 contacts docking surface350 of inventory holder 30, mobile drive unit 20 begins liftinginventory holder 30 off the ground. As a result, in such embodiments,mobile drive unit 20 supports inventory holder 30 once mobile drive unit20 is docked with inventory holder 30.

At step 808, mobile drive unit 20 moves inventory holder 30 towards adestination requested by the received command. As noted above, whilemoving, mobile drive unit 20 may continually monitor and/or periodicallymake an updated determination of the location of mobile drive unit 20and/or the relative position of mobile drive unit 20 and inventoryholder 30. In particular embodiments, mobile drive unit 20 may monitoror determine the location of mobile drive unit 20 by detecting fixedobjects, such as fiducial markers 50, as mobile drive unit 20 moves. Atstep 810, mobile drive unit 20 determines that mobile drive unit 20 hasreached the requested destination. In particular embodiments, mobiledrive unit 20 may determine that mobile drive unit 20 has reached therequested determination by detecting a fixed object, such as a fiducialmarker 50 associated with the requested destination.

Upon reaching the requested destination, mobile drive unit 20 maydetermine the location of inventory holder 30 at step 812. As notedabove, mobile drive unit 20 may include a holder sensor 150 that iscapable of detecting some portion or feature of inventory holders 30,such as a holder identifier 360, that holder sensor 150 can use todetermine the location of the relevant inventory holder 30 relative tomobile drive unit 20. Mobile drive unit 20 may then calculate, at step814, a difference between the location of inventory holder 30 and therequested destination. At step 816, mobile drive unit 20 may determinewhether the difference is greater than a predetermined tolerance.

If the difference is not greater than a predetermined tolerance, mobiledrive unit 20 may undock from inventory holder 30 at step 818. Inparticular embodiments, mobile drive unit 20 may undock by lowering adocking head 110 that is supporting inventory holder 30. Mobile driveunit 20 may then move away from inventory holder 30, at step 820, andproceed with responding to other commands. The operation of mobile driveunit 20, with respect to moving this inventory holder 30, may then end,as shown in FIG. 7.

If, instead, the difference between the actual location of inventoryholder 30 and the requested destination is greater than thepredetermined tolerance, in particular embodiments, mobile drive unit 20may take appropriate remedial action to correct the misalignment. Forexample, in the described embodiment, mobile drive unit 20 calculates acorrection value based on the difference between the requesteddestination and the actual location of inventory holder 30 at step 822.In particular embodiments, the correction value may represent a distanceand/or direction in which to adjust the location of inventory holder 30to correct for the detected misalignment.

Additionally, in the described embodiment, mobile drive unit 20determines an adjusted destination using the correction value at step824. For example, if mobile drive unit 20 determines upon arriving atthe requested destination that the relevant inventory holder 30 hasshifted during transport and is now a quarter inch to the east of itsanticipated location while docked with mobile drive unit 20, mobiledrive unit 20 may determine an adjusted destination that is a quarterinch west of the requested destination specified by the command. Mobiledrive unit 20 may then move mobile drive unit 20 and inventory holder 30to the adjusted destination at step 826.

At step 828, mobile drive unit 20 may then undock from inventory holder30 at the adjusted destination. In particular embodiments, mobile driveunit 20 may take additional steps to detect and/or correct any remainingor subsequently occurring misalignment of inventory holder 30 afterarriving at the adjusted destination and/or after undocking to furtherreduce the potential for misalignments. For example, in particularembodiments, mobile drive unit 20 may, after undocking, attempt todetect and correct any misalignments using techniques similar to thosedescribed above in FIG. 5. Mobile drive unit 20 may then move away frominventory holder 30, at step 830, and proceed with responding to othercommands. The operation of mobile drive unit 20, with respect to movingthis inventory holder 30, may then end, as shown in FIG. 6.

By determining whether inventory holder 30 is misaligned beforeundocking, particular embodiments of mobile drive unit 20 may be able tolimit the time and power expended in correcting such misalignments. Inparticular embodiments, this may reduce the time required to respond toorders and/or limit the frequency with which mobile drive units 20 mustrecharge its power supply. As a result, embodiments of mobile drive unit20 that operate as described by FIG. 7 may provide a number ofoperational benefits.

FIG. 8 illustrates another example technique that may be utilized bycertain embodiments of mobile drive unit 20 in detecting and correctingmisalignments of inventory holder 30. In particular, FIG. 8 illustratesa technique in which mobile drive unit 20 determines, while in transitto a requested destination, that inventory holder 30 is misaligned andcorrects for this misalignment before arriving at the requesteddestination. Some of the steps illustrated in FIG. 8 may be combined,modified, or deleted where appropriate, and additional steps may also beadded to the flowchart. Additionally, steps may be performed in anysuitable order without departing from the scope of the invention.

In this embodiment, operation begins at step 900 with mobile drive unit20 receiving a command identifying an inventory holder 30 to be moved ora location for an inventory holder 30 to be moved. In response toreceiving the command, mobile drive unit 20 moves to or near thestarting location for the identified inventory holder 30 at step 902. Inparticular embodiments, mobile drive unit 20 may position mobile driveunit 20 under the identified inventory holder 30, at step 904, orperform any other adjustments to align mobile drive unit 20 withinventory holder 30, prepare mobile drive unit 20 or inventory holder 30for docking, and/or otherwise facilitate docking and transport.

Mobile drive unit 20 may then initiate docking at step 906. Inparticular embodiments, mobile drive unit 20 initiates docking byraising docking head 110. When docking head 110 contacts docking surface350 of inventory holder 30, mobile drive unit 20 begins liftinginventory holder 30 off the ground. As a result, in such embodiments,mobile drive unit 20 supports inventory holder 30 once mobile drive unit20 is docked with inventory holder 30. At step 908, mobile drive unit 20moves inventory holder 30 towards a destination requested by thereceived command.

At some point while moving toward the requested destination, mobiledrive unit 20 may determine the location of inventory holder 30 relativeto mobile drive unit 20, as shown at step 910. For example, inparticular embodiments, mobile drive unit 20 may be configured toperiodically check the location of inventory holder 30 relative tomobile drive unit 20 while moving inventory holder 30 to the requesteddestination. Mobile drive unit 20 then calculates, at step 912, adifference between the location of inventory holder 30 and itsanticipated location while docked with mobile drive unit 20. Inparticular embodiments, mobile drive unit 20 may be programmed withinformation specifying the anticipated location of a particular portionof inventory holder 30 (such as holder identifier 360) relative tomobile drive unit 20 when inventory holder 30 is docked and properlyaligned with mobile drive unit 20. In such embodiments, mobile driveunit 20 may calculate the difference between the location of inventoryholder 30 and its anticipated location based on the difference betweenthe location of the relevant portion and the anticipated location ofthat portion of inventory holder 30.

At step 914, mobile drive unit 20 then determines whether the differencebetween the actual location of inventory holder 30 and the anticipatedlocation of inventory holder 30 is greater than a predeterminedtolerance. If the difference is not greater than the predeterminedtolerance, operation of mobile drive unit 20 continues at step 924. If,instead, the difference is greater than the predetermined tolerance,mobile drive unit 20 may determine a correction value based on thisdifference at step 916. As noted above, mobile drive unit 20 may, inparticular embodiments, be programmed with information specifying theanticipated location of a particular portion of inventory holder 30(such as holder identifier 360) relative to mobile drive unit 20. Mobiledrive unit 20 may determine, while moving, the location of this portionof inventory holder 30 and, if the location of the relevant portiondiffers from its anticipated location by more than the predeterminedtolerance, determine an appropriate correction value based on themagnitude and/or direction of the misalignment. For example, if mobiledrive unit 20 determines that the relevant portion of inventory holder30 is a quarter inch to the east of the anticipated location for thisportion of inventory holder 30, mobile drive unit 20 may determine acorrection value that is a quarter inch to the west.

Mobile drive unit 20 then undocks from inventory holder 30 at step 918.At step 920, mobile drive unit 20 moves a distance and/or directiondetermined based on the correction value. Returning to the quarter-inchmisalignment example, mobile drive unit 20, in accordance with thecorrection value calculated for this misalignment, moves a quarter inchto the west to correct the misalignment. Then, after completing anyappropriate movements based on the correction value, mobile drive unit20 re-docks with inventory holder at step 922.

After correcting any existing misalignment or determining that anymisalignment is less than the predetermined tolerance, mobile drive unit20 may continue moving toward the requested destination at step 924. Atstep 926, mobile drive unit 20 may then determine that mobile drive unit20 has reached the requested destination. Mobile drive unit 20 thenundocks from inventory holder 30 at step 928. After undocking, mobiledrive unit 20 may then move away from inventory holder 30 and respond toother commands at step 930. Operation of mobile drive unit 20 withrespect to moving this particular inventory holder 30 may then end asshown in FIG. 8.

Although the present invention has been described with severalembodiments, a myriad of changes, variations, alterations,transformations, and modifications may be suggested to one skilled inthe art, and it is intended that the present invention encompass suchchanges, variations, alterations, transformations, and modifications asfall within the scope of the appended claims.

1. A method for transporting inventory items, comprising: moving amobile drive unit to a first point while the mobile drive unit is dockedwith an inventory holder; determining a location of the inventory holderwhile the mobile drive unit is docked with the inventory holder;calculating a difference between the location of the inventory holderand the first point; determining whether the difference is greater thana predetermined tolerance; in response to determining that thedifference is less than the predetermined tolerance, undocking from theinventory holder; and in response to determining that the difference isgreater than the predetermined tolerance: moving the mobile drive unitto a second point based on the location of the inventory holder; andundocking the mobile drive unit with the inventory holder.
 2. The methodof claim 1, wherein the moving the mobile drive unit to a second pointcomprises: determining an adjusted location based, at least in part, onthe location of the inventory holder; and moving the inventory holder tothe adjusted location.
 3. The method of claim 1, wherein determining thelocation of the inventory holder while the mobile drive unit is dockedwith the inventory holder comprises determining a location of the mobiledrive unit and determining a position of at least part of the inventoryholder relative to the location of the mobile drive unit.
 4. The methodof claim 3, wherein determining the location of the mobile drive unitcomprises: detecting a fixed object; and determining the location of themobile drive unit based on the fixed object.
 5. The method of claim 1,wherein determining the location of the inventory holder while themobile drive unit is docked with the inventory holder comprises:detecting a holder identifier associated with the inventory holder; anddetermining the location of the inventory holder based on a position ofthe holder identifier relative to a location of the mobile drive unit.6. The method of claim 1, further comprising determining that the mobiledrive unit has reached the first point based on detecting a fixed objectassociated with the first point.
 7. An apparatus comprising: a drivemodule operable to propel the apparatus towards a destination; a dockingactuator operable to facilitate docking with an inventory holder; acontrol module operable to: cause the apparatus to move to a first pointwhile the apparatus is docked with an inventory holder; determine alocation of the inventory holder while the apparatus is docked with theinventory holder; calculate a difference between the location of theinventory holder and the first point; determine whether the differenceis greater than a predetermined tolerance; in response to determiningthat the difference is less than the predetermined tolerance, cause theapparatus to undock from the inventory holder; and in response todetermining that the difference is greater than the predeterminedtolerance: cause the apparatus to move to a second point based on thelocation of the inventory holder; and cause the apparatus to undock withthe inventory holder.
 8. The apparatus of claim 7, wherein the controlmodule is operable to cause the apparatus to move to the second pointby: determining an adjusted location based, at least in part, on thelocation of the inventory holder; and causing the apparatus to move theinventory holder to the adjusted location.
 9. The apparatus of claim 7,wherein the control module is operable to determine the location of theinventory holder while the apparatus is docked with the inventory holderby: determining a location of the apparatus; and determining a positionof at least part of the inventory holder relative to the location of theapparatus.
 10. The apparatus of claim 9, wherein the control module isoperable to determine the location of the apparatus by: detecting afixed object; and determining the location of the apparatus based on thefixed object.
 11. The apparatus of claim 7, wherein the control moduleis operable to determine the location of the inventory holder while theapparatus is docked with the inventory holder by: detecting a holderidentifier associated with the inventory holder; and determining thelocation of the inventory holder based on a position of the holderidentifier relative to a location of the apparatus.
 12. The apparatus ofclaim 7, the control module further operable to determine that theapparatus has reached the first point based on detecting a fixed objectassociated with the first point.
 13. The apparatus of claim 7, furthercomprising means for determining a location of the apparatus.
 14. Asystem for storing inventory items, comprising: an inventory holderoperable to store inventory items; and a mobile drive unit operable to:move to a first point while docked with an inventory holder; determine alocation of the inventory holder while docked with the inventory holder;calculate a difference between the location of the inventory holder andthe first point; determine whether the difference is greater than apredetermined tolerance; in response to determining that the differenceis less than the predetermined tolerance, undock from the inventoryholder; and in response to determining that the difference is greaterthan the predetermined tolerance: move to a second point based on thelocation of the inventory holder; and undock with the inventory holder.15. The system of claim 14, wherein the mobile drive unit is operable tomove to the second point by: determining an adjusted location based, atleast in part, on the location of the inventory holder; and moving theinventory holder to the adjusted location.
 16. The system of claim 14,wherein the mobile drive unit is operable to determine the location ofthe inventory holder while docked with the inventory holder by:determining a location of the mobile drive unit; and determining aposition of at least part of the inventory holder relative to thelocation of the mobile drive unit.
 17. The system of claim 16, whereinthe mobile drive unit is operable to determine the location of themobile drive unit by: detecting a fixed object; and determining thelocation of the mobile drive unit based on the fixed object.
 18. Thesystem of claim 14, wherein the mobile drive unit is operable todetermine the location of the inventory holder while docked with theinventory holder by: detecting a holder identifier associated with theinventory holder; and determining the location of the inventory holderbased on a position of the holder identifier relative to a location ofthe mobile drive unit.
 19. The system of claim 14, the mobile drive unitfurther operable to determine that the mobile drive unit has reached thefirst point based on detecting a fixed object associated with the firstpoint.
 20. The system of claim 14, further comprising means fordetermining the location of the mobile drive unit.